Latches in digital circuits can be formed in many ways, however, one common way to create a latch within CMOS integrated circuits is to use a pair of inverters connected back to back, as shown in FIG. 1. As the circuit of FIG. 1 shows, when control signal 106 is high, it enables NFET 108, which connects data signal 104 to the input of inverter 110. Inverter 110 inverts the signal on its input to produce an output signal 116, which is inverted again by inverter 112 as a feedback signal to cause the circuit to latch. Inverter 110 is called the feed-forward inverter, and inverter 112 is called the feedback inverter.
For the circuit of FIG. 1 to work properly, the feedback inverter 112 must be a smaller inverter, thus having a weaker drive output, then the feed-forward inverter 110. This is necessary so that the data 104 can overdrive the input signal 114 to force the input 114 into the correct state in order to change the latch from one state to another. If the feedback inverter 112 is too weak, however, the circuit may be unstable.
There are two important characteristics of such a latch, gain and resolving time. Gain, of course, determines how much input signal is required to cause the inverter to invert the input signal to produce an inverted output signal. The resolving time, .tau., is a point at which the output equals sixty-three percent of its original value. The two primary effects on .tau. are the gain of the circuit and the capacitance of the circuit, wherein the smaller the gain and the larger the capacitance the slower the circuit will resolve. The gain can be improved by putting more strength into the feedback loop. At some point, however, the feedback inverter 112 becomes so strong that it cannot be overdriven by the data signal 104 in order to change the state of the latch.
There is need in the art then for a method of creating a latch circuit with a stronger feedback signal to improve the resolving time, while still allowing the feedback signal to be overdriven by input data. The present invention meets this and other needs in the prior art.